This invention relates to cathode ray tubes (CRTs) and, more particularly, to CRTs in which the target is made of single crystal yttrium aluminum garnet (YAG; Y.sub.3 Al.sub.5 O.sub.12).
In U.S. Pat. No. 4,514,756 assigned to the assignee hereof, S. L. Blank and E. I. Gordon describe a target for a single beam color CRT. The target as shown in FIG. 2 of the patent and reproduced as FIG. 1 herein, comprises a plurality of color stripe triads: layer R for generating red light, layer G for generating green light, and layer B for generating blue light. These stripes are arranged on a substrate 14 in a staircase geometry so that, as viewed by the e-beam 12, they appear as side-by-side color stripes. The particular arrangement with the red stripe on the top and the blue stripe on the bottom is illustrative only--any sequence of the three primary colors is suitable.
Overlaying each stripe triad is a metal layer. Two metal layers designated 20.1 and 30.1 are depicted as overlaying adjacent triads. The e-beam 12 is modulated with video (color) information and made to scan horizontally (i.e., transversely) across the electrodes in the direction, for example, of arrow 17. The metal layers do not stop the electrons which penetrate into the stripes R, G and B. The e-beam energy is chosen, however, so that the electrons are absorbed in only the uppermost color stripe. Thus the beam produces only a single primary color at a time (i.e., in a given beam position). For example, in the figure, for the beam position shown, the electrons are absorbed in the green stripe G and generate light of wavelength .lambda..sub.G, but they do not have enough energy to penetrate into the underlying blue stripe B. The light intensity is modulated by modulating the e-beam current as is well-known in the art.
In a high-brightness CRT, of the type suitable for a projection CRT system, the target comprises a YAG substrate 14 on which are epitaxially grown three YAG layers, each one producing a primary color for use in color display. Illustratively the R layer comprises Eu:YAG, the G layer comprises Ce:YAG and the B layer comprises Bi:YAG. These layers are then etched or otherwise suitably shaped to form the staircase configuration.
YAG has been etched by a number of techniques in the prior art. Reactive sputter etching is described briefly in U.S. Pat. No. 4,298,820 granted to P. F. Bongers et al on Nov. 3, 1981. However, sputter etching involves an extremely slow etch rate (approximately 14 .ANG./min) and, therefore, is impractical for large scale production. On the other hand, wet chemical etching using a complicated mask structure is described by M. Berkenblit et al in U.S. Pat. No. 4,106,975 issued on Aug. 15, 1978. The Berkenblit process, which is directed to etching pyramid-like holes in YAG for fluid spray nozzles in jet printing applications, involves five layers of masking material, one heat treatment (temperatures between 700.degree. C. and 1100.degree. C. for between 15 minutes and 2 hours), and three wet chemical etching steps. This process is complex, time consuming, costly and not practical for large scale manufacturing either. A simpler mask and etching process are clearly desirable.
Although a simple layer of SiO.sub.2 is a common mask for etching semiconductors, it is not known to utilize it as an etching mask directly on YAG. Indeed the disparate chemical properties of semiconductors and YAG strongly suggest that the substitution is not an obvious one. This conclusion is supported by the Berkenblit et al patent (FIG. 1) which utilizes a composite three-layer metal mask of chromium 2, platinum 3, and chromium 4 to etch a YAG substrate 1, but uses an SiO.sub.2 layer 5 only as a mask to etch the stack of metal layers. For chemical reasons related to adhesion, the SiO.sub.2 layer 5 is not deposited directly on the YAG substrate nor is it even present when the YAG is being etched. (See column 6, lines 15-19 of the Berkenblit et al patent.)
As noted in Berkenblit et al at column 3, lines 32-34, the ability to etch a material such as YAG is a very complex and empirical art. The unpredictability of the action of a particular etchant mixture and the suitability of mask materials, in combination with the importance placed by Berkenblit et al on the use of a composite metal mask to etch YAG, make it clear that it is not obvious to use SiO.sub.2 directly on YAG as a mask to etch stripes for a color CRT target.